How do i remove a chunk of text from a variable? (PHP)

[ssmol]

I need to be able to remove a URL from a variable, I'm wondering how i do this. Example - Say my script returns http://www.example.com/file.php?id=1234 i need to be able to remove the http://www.example.com/file.php?id= bit, just leaving the id number. If anyone can help, it would be great